Field of the Invention
The present invention relates to a protective film forming method for forming a protective film on the front side of a wafer such as a semiconductor wafer.
Description of the Related Art
As well known in the art, in a semiconductor device fabrication process, a plurality of crossing division lines are formed on the front side of a semiconductor substrate such as a silicon substrate to thereby define a plurality of separate regions where a plurality of devices such as integrated circuits (ICs) and large scale integrations (LSIs) are each formed. Such a wafer having the plural devices formed on the front side of the semiconductor substrate is cut along the division lines to thereby obtain individual device chips. Further, in an optical device wafer, the front side of a sapphire substrate or the like is partitioned into a plurality of separate regions by a plurality of crossing division lines, and a gallium nitride compound semiconductor or the like is layered in each separate region of the substrate to thereby form an optical device. Such an optical device wafer is divided along the division lines to obtain individual optical device chips such as light emitting diodes and laser diodes, which are widely used for electrical equipment.
As a method of dividing a wafer such as a semiconductor wafer and an optical device wafer along the division lines formed on the front side of the wafer, there has been proposed a method including the steps of applying a pulsed laser beam to the wafer along the division lines to thereby form a plurality of laser processed grooves and next breaking the wafer along these laser processed grooves by using a mechanical breaking apparatus.
Such laser processing has advantages over cutting such that a processing speed is higher and a wafer formed of a hard material such as sapphire can be processed relatively easily. However, when a laser beam is applied to the wafer along the division lines, thermal energy is concentrated at a region irradiated with the laser beam, causing the generation of debris, and this debris may stick to the surface of the devices formed on the wafer, causing a degradation in quality of the devices.
To solve this problem due to the debris, there has been proposed a laser processing apparatus having a configuration such that a protective film of resin such as polyvinyl alcohol (PVA) is formed on the front side (work surface) of a wafer and a laser beam is next applied through the protective film to the wafer (see Japanese Patent Laid-open No. 2004-322168, for example).
In the laser processing apparatus disclosed in Japanese Patent Laid-open No. 2004-322168, the protective film is formed by a so-called spin coating method including the steps of holding the wafer on a spinner table, supplying a water-soluble resin such as PVA to the center of the front side of the wafer held on the spinner table, and rotating the spinner table to thereby spread the water-soluble resin toward the outer circumference of the wafer due to a centrifugal force, thus forming the protective film on the front side of the wafer.